Hcompress Image
Compression Software




Hcompress is the image compression package written by Richard L. White for use at the Space Telescope Science Institute. Hcompress was used to compress the STScI Digitized Sky Survey and has also been used to compress the preview images in the Hubble Data Archive. Briefly, the method used is:

The technique gives very good compression for astronomical images and is fast, requiring about 1 second for compression or decompression of a 512x512 image on a Sun SPARCstation 2. The calculations are carried out using integer arithmetic and are entirely reversible. Consequently, the program can be used for either lossy or lossless compression, with no special approach needed for the lossless case (e.g. there is no need for a file of residuals.) Documentation on the use of the programs and on the compression method is included in the doc subdirectory.

These programs can compress 2-byte integer images in a small variety of input formats (including FITS format, the most widely used format for astronomical images.) The resulting compressed image file is machine-independent and may be transferred between computers with binary FTP. The source code is reasonably machine-independent, although it has not been tested on a wide range of machines. Installation instructions for Unix and VMS (VAX) machines are given in the README files in the appropriate subdirectories. I would be very interested to hear of any attempts to install this code on other kinds of machines (e.g. MS-DOS, Macs.)


This directory has two files that contain the hcompress image compression package:

This version (28 February 1994) corrects one bug in the decompression from the previous version (20 April 1992). The effect of this bug was only noticeable for lossless compression of odd-sized images (images with dimensions that are not a power of 2.) The compressed files in that case were correct, but the decompressed images could have pixel values that differed by +/- 1 count compared to the original. If you have any old compressed files, they will decompress properly using this new version of the software. Only hinv.c has changed from the previous version. I know of no other bugs that have been discovered (but would welcome reports of such.)


Get Unix Compressed Tar File

[Link] File hcompress.tar.Z

Get VMS Files

[Link] File hcompress.bck
[Link] File backup.com
[Link] File modify.exe


Copyright (c) 1994 Association of Universities for Research in Astronomy. All rights reserved. Produced under National Aeronautics and Space Administration Contract No. NAS5-26555.
STScI Home Page | Software
Richard L. White, rlw@stsci.edu
1997 April 23